1. Field of the Invention
The present invention relates to a semiconductor optical amplifier—Mach-Zehnder interferometer (SOA-MZI). More particularly, the present invention relates to a method of controlling at least one of an optical gain difference and an optical phase difference, particularly to optimize an extinction ratio (ER), and apparatuses appropriate therefore.
2. Description of the Related Art
A broadband information communication network having a large capacity, increased transmission speed, increased operational efficiency and improved reliability is needed. Wavelength division multiplexing (WDM) transmission is one intercommunication technology that meets the above needs. WDM not only increases the transmission capacity, but also establishes a reconfigurable network. In a WDM network, data is transmitted to nodes along a path that is determined by a wavelength used. However, due to connections between networks using different wavelengths and/or a limit on the number of wavelengths which can be used per channel in a network, wavelengths may interfere with each other as the same wavelength is used in different channels, or assignment of a path to each wavelength and efficient routing is made difficult, giving rise to problems in the system.
Accordingly, to efficiently operate a WDM all-optical network, optical exchange is needed at each node to avoid channel conflict and to provide wavelength reallocation. Such optical exchange may be performed by an optical cross connect (OXC). The OXC consists of a wavelength converter converting an input signal at one wavelength to an output signal at another wavelength, a spatial switch routing paths of wavelengths, an optical add/drop multiplexer (OADM) adding or dropping a path of an optical signal that is input or output at a node, and a MUX/DEMUX coupling signals in various wavelengths or separating a coupled signal. Of the above devices, the wavelength converter is a core device of the OXC. The conversion of a wavelength allows incoming data carried on a particular wavelength to be transmitted using a newly assigned wavelength. The wavelength conversion may rely on cross-gain modulation (XGM) using non-linearity of a semiconductor optical amplifier (SOA), cross-phase modulation (XPM) and four wave-mixing (FWM). A SOA-MZI wavelength converter uses XPM.
In general, an SOA is an optical amplifier that, when an input signal having a small amplitude is incident on an active layer of a semiconductor in a state in which a density is inverted by the injection of current, the input signal is amplified by stimulated emission in which free electrons in a conduction band are coupled to holes in a valence band, thereby emitting photons. Thus, an amplified optical signal is output.
FIG. 1 illustrates a block diagram showing the structure of a conventional SOA-MZI wavelength converter. Referring to FIG. 1, the SOA-MZI wavelength converter converts the wavelength of an input optical signal from λ1 to λ2 using XPM. The SOA-MZI wavelength converter shown in FIG. 1 includes a first SOA 102, a second SOA 104, a phase shifter 106, and an optical bandwidth pass filter (OBPF) 108.
The first SOA 102 amplifies a pump input signal Ppump and a probe input signal Pprobe. When Ppump is logic 1, the first SOA 102 also shifts the phase of Pprobe due to XPM. The optical gain of the first SOA 102 is controlled by a bias current i1. In the SOA-MZI wavelength converter shown in FIG. 1, a portion amplifying the pump input signal Ppump and the probe input signal Pprobe using the first SOA 102 is referred to as a first arm. The second SOA 104 amplifies the probe input signal Pprobe. The optical gain of the second SOA 104 is controlled by a bias current i2. In the SOA-MZI wavelength converter shown in FIG. 1, a portion amplifying the probe input signal Pprobe using the second SOA 104 and shifting the phase of an amplified optical signal output from the SOA 104 using the phase shifter 106 is referred to as a second arm.
The phase shifter 106 changes an optical phase difference φ between the first and second arms to π in order to increase an extinction ratio (ER). When the optical phase difference φ between the first and second arms is π and an optical phase difference φXPM generated due to Pprobe being logic 1 is π, constructive interference occurs. When the optical phase difference φ between the first and second arms is π and probe input signal Pprobe is logic 0, i.e., φXPM is 0, destructive interference occurs. Thus, ER is optimized.
The OBPF 108 cuts off a pump output signal at λ1 and passes only a probe output signal at λ2. Thus, only a probe output signal having a wavelength of λ2 modulated to the shape of a pump signal is transmitted, thereby achieving wavelength conversion.
FIGS. 2A–2C illustrate waveform diagrams showing the operation of the apparatus shown in FIG. 1. In FIG. 2A, a waveform diagram shows a pump input signal. The pump input signal may be an intensity modulated (IM) optical signal at λ1, i.e., has a binary pulse. In FIG. 2B, a waveform diagram shows a probe input signal. The probe input signal may be a continuous wave (CW) optical signal at λ2, e.g., a laser signal. In FIG. 2C, a waveform diagram shows a probe output signal output from the OBPF 108. The probe output signal is a signal having a wavelength of λ2 and is intensity-modulated to a pulse form identical to the pump input signal, i.e., an IM optical signal having a wavelength converted from λ1 to λ2.
Referring to the waveforms illustrated in FIGS. 2A–2C, the apparatus shown in FIG. 1 converts an IM optical signal at λ1 to an IM optical signal at λ2, i.e., converts the wavelength.
FIG. 3A illustrates a graph of optical gains relative to input power of the first and second SOAs 102 and 104 shown in FIG. 1. Referring to FIG. 3A, the optical gains of the first and second SOAs 102 and 104 maintain particular values until a threshold value, indicated by a dotted line, and decrease at a particular inclination after the threshold value, demonstrating a gain saturation characteristic.
FIG. 3B illustrates a graph of a phase difference relative to input power between the two arms of the SOA-MZI wavelength converter shown in FIG. 1. Referring to FIG. 3B, the phases of the two SOAs 102 and 104 characteristically change at the threshold value. That is, when the amplitudes of signals input to the two SOAs 102 and 104 are less than a threshold value or Ppump is logic low, the phase difference is simply equal to a phase shift introduced by the phase shifter 106, here π. When the signal amplitudes exceed the threshold value and Ppump is logic high, then the phase difference is the sum of φ and φXPM, which may be up to ±π.
In the SOA-MZI wavelength converter of FIG. 1, the phase of the second arm is delayed relative to the first arm by π due to the phase shifter 106. When the amplitude of the pump input signal is lower than the threshold value, no phase delay occurs in the first SOA 102. When the amplitude of the pump input signal is higher than the threshold value, an optical phase difference φXPM of up to ±π may be generated due to XPM.
The pump input signal is binary. Thus, in the SOA-MZI wavelength converter, considering φXPM, the optical phase difference φ between the first and second arms is π when the pump input signal is logic 0 and is 0 when the pump input signal is logic 1. Hence, when the pump input signal is logic 1, the optical power levels of the probe output signals interfere in accordance with the sum of φ and φXPM, and, when the pump input signal is logic 0, the optical power levels of the probe output signals interfere in accordance with φ. Thus, the optical power levels of the probe output signal are modulated to be identical to the logic level of the pump input signal.
That is, in the probe output signal, constructive interference results when a total optical phase difference is zero or an integer multiple of 2π and destructive interference results when the total optical phase difference φ is (2n+1)π. Thus, the wavelength conversion is performed by the interference and the OBPF 108 so that a signal having a wavelength of λ1 is changed to a signal having a wavelength of λ2 and a probe output signal POprobe shown in FIG. 2C is generated.
The SOA-MZI wavelength converter according to the conventional technology has a high ER and outputs a non-inverted signal. However, the phase shift arising from XPM is generated in a small range of a high pump input signal, i.e., in a small range at a high voltage level. Thus, the optical phase difference φXPM decreases as the power of the pump input signal decreases. Accordingly, an input power dynamic range (IPDR) of the pump input signal decreases.
Also, a high ER can be maintained by accurately adjusting the optical phase difference φ between the two arms to be π. For conventional technology to achieve this, the phase shifter 106 needs to be manually adjusted. However, it is difficult to manually adjust the phase shifter 106 to make the optical phase difference φ between the two arms be exactly π. It is further difficult to manually adjust the π phase shifter 106 to take into account that the phase shift due to XPM is affected by the power of the pump input signal.
The difficulty in controlling the optical phase difference φ between the two arms to be π is further increased, since the optical phase difference φ is very sensitive to external environmental factors, e.g., the operational temperature of the SOA.